The role of cell cycle in reprogramming toward induced pluripotent stem cells (iPSCs)

Abstract Effective generation of human induced pluripotent stem cells (hiPSCs) holds great promise for their clinical application and drug screening, as well as for in vitro modeling of human disease and understanding early steps of human preimplantation development. Generation of iPSCs also presents a powerful tool for dissecting mechanisms underlying the establishment of pluripotency. However, nearly 14 years since the first hiPSC line was generated, our understanding of the molecular mechanisms governing conversion of specialized somatic cells back to pluripotency remains poor. One approach to elucidate this issue would constitute in comparing main differences between somatic and pluripotent cells. As such, cell cycle structure and its molecular machinery appear to be completely different between these cells. Hence, this review will address key differences between an abbreviated cell cycle characteristic of embryonic stem cells, as compared with that of somatic cells. In addition, we will highlight differences in molecular organization of the cell cycle of mouse and human embryonic stem cells (hESCs) with the special focus on G1, S, and G2/M-phase regulation. Remodeling of the somatic cell cycle structure for the stem-cell-like type is one of the earliest important steps for efficient reprogramming, and it will be discussed in the context of mouse and hiPSCs generation. Why acquisition of a stem-cell-like cell cycle profile and a high proliferation rate are so important? To answer this question, this review will focus on the current knowledge about the role of major cell cycle driving forces, such as Cyclin/Cdk complexes and Rb, as well as negative regulators of the cell cycle, such as cell cycle inhibitors, members of the Cip/Kip, and Ink families in the stem cell maintenance and pluripotency acquisition. Special attention will be dedicated to the question of cell fate commitment as a function of cell cycle stage and to the role of S-phase molecular machinery in pluripotency and reprogramming. We will also review p53 pathway, which is considered to be an important guardian of the genome with a well-defined cell cycle function. Indeed, p53 pathway is activated by reprogramming factors and is also known as a barrier of induced pluripotency. However, how safe is it to downregulate this pathway for better effectiveness of reprogramming? Evidence is presented that from the point of view of clinical-grade “safe” hiPSCs, this approach is not viable, as such iPSCs are prone to genome instability. With regard to iPSCs “safety” downregulation of other reprogramming barriers, such as Cip/Kip/Ink cell cycle inhibitors will be also revised. At that part of the review, we discuss the role of the Ink4/Arf locus or Cip/Kip cell cycle inhibitors aiming to explain why deletion of p27 or p18, rather than loss of p21, may be a better choice to enhance iPSC generation. Hence, identifying defining factors and key mechanisms that regulate proliferation kinetic and stem-cell-like cell cycle structure during the cellular reprogramming toward iPSCs presents a major advantage for future research. Reviewing recent advances in cell cycle molecular mechanisms involved in reprogramming to pluripotency is essential not only for further improvement of iPSC technology but also for our deeper understanding of stem cell biology.